This invention relates generally to computed tomography (CT) imaging and more particularly, to measuring curved distances of an object of interest utilizing maximum intensity projection (MIP) images.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
At least one known CT imaging system is configured to perform CT angiograghy (CTA). Compared to a conventional x-ray angiogram, the CTA system is advantageous as result of being non-invasive and more cost effective. In performing a CTA procedure, a set of CT images are first acquired and a Maximum Intensity Projection (MIP) image is generated to mimic the appearance of an x-ray angiogram. To measure the length of a section, or segment, of a vessel, several points are identified in the MIP image. A set of straight line distances between two points are then measured. For straight vessels that are perpendicular to the direction of projection, such a procedure yields satisfactory results. However, known CTA systems are unable to accurately measure a distance of a vessel segment being curved in a direction non-parallel to a MIP projection plane. As a result, known CTA systems typically underestimate the length of the vessel.
At least one known x-ray angiography system overcomes the difficultly of measuring curved vessels by placing a specially designed catheter inside the vessel. The catheter includes a series of uniformly spaced beads to provide distance markers on the angiography images. The distance between two points is determined by counting the number of beads in the x-ray image. However, as described above, such x-ray systems are invasive and very costly.
It would be desirable to provide an system which facilitates accurate measurement of vessel segment. It would also be desirable to provide such a system which facilitates retrospective selection of any vessel segment.